Posted Saturday, March 10/07 in Mods & Tests

I went snowboarding a few weekends back (with 5 other people via diesel van), and couldn't help thinking about aerodynamics & efficiency.

Partly that's because I'm a lousy snowboarder. To keep from coming to a dead stop on the flat sections of the (easy) runs I tend to ride, I sometimes have to crouch way down low (minimizing my frontal area) to maintain to enough momentum to "coast" to where the run drops off again.

But mostly it was because of what I saw in the ski hill parking lot: a sea of roof racks and roof-top carriers. I wondered how many people were aware of the magnitude of the fuel consumption penalty they cause. I wasn't entirely sure myself, so I did a quick comparison and saw some dramatic results first-hand.

Roof rack test overview ...

• The 9th commandment
  
• The (not very rigorous) test: A-B-C
  
• Roof rack results
  
• Observations... move that commandment!
  
• Anectodal corroboration
  
• Carrying options
  
• Resources

The 9th commandment ...

Read almost any list of fuel efficient driving tips, and usually way down near the bottom you'll almost invariably find advice to "remove roof racks when not in use."

Despite seeing this point made regularly, they never provide enough information on just how big a deal it is. The stated MPG penalty isn't consistent (if it's stated), and when it is stated, it varies wildly (eg. 5% to 20% mentioned in 2 of the above links).

So I decided to do a quick experiment to see what I could learn.

The (not very rigorous) test: "A-B-C" ...

I did these roof rack runs as part of the warm-up for last year's side mirrors experiment.

The comparison was between:

• A - no roof racks
  
• B - empty roof racks
  
• C - a mountain bike on the roof racks

It should be said this wasn't a rigorous test. I only did one bi-directional run for each of the B and C conditions. So, while the results were dramatic, you'll have to take them with a grain of salt large enough to require roof-top transport.

Conditions ...

Air Temperature (ATMP).. 67.6 F / 19.8 C; Aug 23/06; 7:30 - 8:30 PM
Wind Direction (WDIR)... N ( 0 deg true )
Wind Speed (WSPD)....... 6 kts (11.1 km/h / 6.9 mph)
Wind Gust (GST)......... 7 kts (13.0 km/h / 8.1 mph)
Pressure (IN) .......... 30.01

Results ...

@ 88 km/h (54.7 mph)

A = no roof racks
B = empty roof racks
C = mountain bike in roof racks

A/B/C ... West ... East ... Average MPG (US) / L/100 km / MPG (imperial)

A ..... 55.0 ... 55.3 ... 55.2 / 4.26 / 66.29
A ..... 55.5 ... 55.5 ... 55.5 / 4.24 / 66.65
A ..... 54.8 ... 55.3 ... 55.0 / 4.28 / 66.05
B ..... 49.3 ... 47.3 ... 48.2 / 4.88 / 57.89
C ..... 40.2 ... 40.0 ... 40.1 / 5.87 / 48.16

• Average of all 'A' runs: 55.24 mpg (US)
• Just adding (empty) roof racks reduced fuel economy to 48.2 mpg (US), or -7.0 mpg / -12.7%.
• Putting the mountain bike on the rack absolutely destroyed the mileage! Down to 40.1 mpg (US), or -15.1 mpg / -27.3%

Observations... move that commandment!

Maybe the roof rack warning should be higher up those fuel economy tips lists.

Admittedly, every car/rack combo will be different. But still, wow! I was stunned. I would never have predicted the fuel economy hit would be that big.

But even without the benefit of the ScanGauge telling me what was going on, the extra drag of the mountain bike was immediately obvious to me, by listening to the engine. I run my ignition timing somewhat advanced, which makes the motor more prone to pinging under higher loads. At 55 mph, when the cruise control periodically tugged on the accelerator to maintain the set speed for the test run, I could hear some pinging. Cringe.

But when I think about it, the roof top penalty shouldn't be that surprising. After all, racks both increase frontal area and Cd. The effect is bound to be significant.

That said, not all racks are made equal, and on top of that (pardon the pun) different loads will have different Cd & A characteristics. As you can see in the photos, my racks are definitely "old school", with square tube construction, and slab-faced pillars. Also, I tied the bike down with nylon straps that extended out to the sides and hummed and buzzed in the wind. A dedicated bike carrier (no straps) designed with aerodynamics in mind would undoubtedly perform better.

Still, I'm not the only one who has seen a significant MPG hit from roof-top carrying:

Anectodal corroboration ...

I spoke to a guy who's an avid cyclist who regularly travels to out of town races. He used to carry his bike on top of his Yaris until I showed him the results of my roof rack test.

His next trip was made with the bike on a rear carrier (where the bike is mostly in the car's wake). His results: the best ever fuel economy for a trip, 10% above his lifetime average:

"Best (ever) milage WITH bike on back :)

Well, I took the advice & kept my ride off the top of the car. I bought a hatch mounted setup that works pretty smoothly.

So I put my bike & an extra set of wheels mounted on the back & headed up to (almost) Burlington, Vt for my first cyclocross race of the season (3hrs away).

Almost all highway miles going about 60mph. I filled up on the way back & much to my surprise it calc'd out at a whopping 44.2mpg!! Nice.

I may leave the bike on there a little more. hehe"

Photo courtesy evnut.com

---

From a Prius forum:

"I am not sure about roof racks, but I took my kayak on the roof for a 700 mile trip and my milage dropped from 47 mpg to 36 mpg."

... and in the same thread ...

"Normally I get around 51 without the racks and 48 with the racks (and nothing else)."

---

Even carrying a large load inside a car may prove disastrous (for fuel economy) if it forces you to leave the rear hatch open, as Ecomodder.com member Peakster illustrated. He ran an A-B comparison of hatch-propped open vs. hatch-closed at three different speeds. Here's what he found:

Ecomodder.com member, Peakster, compared the effect of driving with the hatchback open vs. closed.

"Temperature: -4*C
Winds: south @ 11km/h (I was driving west)
Cruise control was used to maintain speeds
Each mpg reading was taken after 2 miles of travel

Hatch closed:
73.6 mpg @ 35 mph
62.9 mpg @ 45 mph
49.2 mpg @ 55 mph

Hatch open:
62.4 mpg @ 35 mph (15.22% mileage loss)
48.2 mpg @ 45 mph (23.37% mileage loss)
34.4 mpg @ 55 mph (30.08% mileage loss)"

Most of the roof-top boxes look aerodynamically backward - they should be blunt up front, and taper toward the rear!

Again, it's not that surprising, when you consider the open hatch both increased frontal area and Cd. (Although it was worse than I predicted.)

---

One more observation: it seems to me the majority of "styled" roof top boxes are aerodynamically backwards. The blunt end should face forward, with the taper at the rear.

Carrying options ...

What do you do, if you have large stuff to transport?

• Carry it inside the car whenever possible.
• If it's too big/awkward to fit inside, carry it behind the vehicle rather than on top.

If you have to carry stuff outside your vehicle, a rear rack is probably the best bet.

• Consider strapping long items under your vehicle. I recently transported an 18 foot mast by strapping it underneath my car (where its effect on Cd is negligible, because the airflow is already turbulent). It was surprisingly easy to strap "up" in a position that was clear of suspension and exhaust components. Just watch out for steep driveway angles!
• If you must carry stuff on top, slow down if possible to minimize the magnitude of the aero hit.
• It would be interesting to compare what's worse: carrying stuff on the roof, or pulling a small, aerodynamic trailer.
• If your vehicle has permanent OEM roof racks, seriously consider making them un-permanent.

Resources ...

• Discussion of roof racks - Ecomodder.com
• Peakster's hatch open/closed mini experiment (with YouTube video) - Ecomodder.com